The benefits of certificate omission schemes in VANET have been so far proven by simulation. However, the research community is lacking of a formal model that would allow implementers and policy makers to select the optimal parameters for such schemes. In this paper, we lay the foundations of the formal model for certificate omission schemes in VANET. We apply the model to ’No Omission’ and ’Periodic Omission’, which validates the previous simulation and formulates the optimal parameters for these schemes

Feiri, Michael

Kargl, Frank

Petit, Jonathan

NARCIS 

Formal Model of Certificate Omission Schemes in VANET

University of Twente Research Information

Formal Model of CertificateOmission Schemes in VANETMichael Feiri∗, Jonathan Petit∗‡, Frank Kargl∗†∗Services, Cybersecurity and Safety, University of Twente, The Netherlands†Institute of Distributed Systems, University of Ulm, Germany‡Department of Computer Science, University College Cork, IrelandAbstract—The benefits of certificate omission schemes inVANET have been so far proven by simulation. However, theresearch community is lacking of a formal model that would allowimplementers and policy makers to select the optimal parametersfor such schemes. In this paper, we lay the foundations of theformal model for certificate omission schemes in VANET. Weapply the model to ’No Omission’ and ’Periodic Omission’, whichvalidates the previous simulation and formulates the optimalparameters for these schemes.Keywords—Analytical model, security, certificate omission,VANET.I. INTRODUCTIONVehicular ad-hoc networks (VANET) enable vehicles to pe-riodically broadcast position beacons, thus providing telematicawareness to neighboring vehicles. The impact of Vehicle-to-Xcommunication (V2X) on safety and traffic efficiency makessecurity mandatory. Therefore, to prevent injection of messagesby external attackers, each vehicle signs every beacon with itsprivate key and appends the certificate to the message. Anyreceiver then has to verify the certificate and the signature ofthe beacon before further processing of the message. Hence,security creates a communication overhead (i.e., packet sizeincreases) and a computational overhead (i.e., time to processthe packet). One approach to reduce overhead is to omitcertificates, decreasing the beacon packet size by 140 bytes [1].Benefits of the certificate omission schemes described belowwere proven by simulation in [2], [3], [4].• No omission of certificates (NoOm): This schemeserves as a baseline as it performs no omission.• Periodic omission of certificates (POoC) [5]: The ideaof POoC is to include a certificate, followed by n− 1omissions, resulting in a periodical pattern of lengthn.• Neighbor-based certificate omission (NbCO) [6]: Thisscheme considers the context of a vehicle in theomission decision. The idea of NbCO is to only attachthe certificate to beacons if there is a change in theneighbor table.• Congestion-based certificate omission (CbCO) [3]:This scheme considers the load of the communicationchannel as the guiding metric. If the communicationchannel is free, there is no need to omit certificates.If the channel is congested, the communication loadis reduced by aggressively omitting certificates.However, a formal model is needed to analytically provewhich scheme is best-suited for VANET, and identify its opti-mal parameters (e.g., n). The main contribution of this paperare the description of the formal model and its application toNoOm and POoC.The rest of the paper is organized as follows. Section IIpresents the system model used for the formal model. Sec-tion III shows the preliminary results of NoOm and POoC.Section IV outlines the future work required to define the fullanalytical model in order to find the optimal parameters forcertificate omission scheme.II. SYSTEM MODELThe goal of modeling certificate omission formally is toanalytically investigate the effect on packet delivery under con-sideration of cryptographic packet loss. Cryptographic packetloss (CPL) in this context is defined as discarding signedmessages that can not be verified cryptographically due to lackof the senders certificate [3]. Without the senders certificate itis impossible to verify the trust relationship between the senderand a trusted certification authority, which is the expected trustmodel for secure vehicular broadcast communication. We referto other forms of packet loss, due to classic signal propagationeffects, as network packet loss (NPL).Packet delivery success generally depends on a multitudeof factors such. Such factors can be the distance betweensender and receiver, the payload length of messages, the loadon the communication channel, shadowing and reflection ofsignals, or environmental aspects such as line of sight. Toremain independent of the intricacies of signal propagationdetails in specific scenarios, we restrict our assumptions aboutthe communication channel to an abstract packet deliveryprobability function Ds(d) for a given scenario s with thedistance d between sender and receiver as input. This func-tion incorporates averaged consideration of above mentionedattributes such as payload length and channel load, includingthe the averaged effects of the selected omissions scheme onthese attributes.Certificate omission achieves lower NPL through a reduc-tion of load in the communication channel. This is obviouslya trade-off against potential CPL. However this CPL effectis only present until the first reception of the certificate ofa sender. Therefore we want to quantify the time until thefirst reception of the certificate of a vehicle with an unknowncryptographic identity. This can occur because a vehicle arriveswithin communication range for the first time or because20114 IEEE Vehicular Networking Conference (VNC)978-1-4799-7660-7/14/$31.00 ©2014 IEEE 41a vehicle switched its cryptographic identity for reasons ofprivacy protection.The predominant communication pattern in vehicular com-munication is expected to be the exchange or position beacons,known as CAM or BSM in the ETSI ITS and IEEE 1609families of standards. These beacons are expected to be sentat a fixed frequency of 10Hz. We use this regular schedule toestablish a baseline metric of beacon periods, which represents100 milliseconds of time. This beacon period allows us to an-alyze network effects based on rounds of message exchanges.Finally we combine the model for the expected time untilthe first reception of a vehicles certificate (CPL) with thegeneral NPL packet loss. This yields the packet receptionprobability at the receiver side under consideration of CPLand NPL for a scenarios packet delivery probability Ds(d) asa function of beacon periods and distance between sender andreceiver.III. PRELIMINARY RESULTSWithout loss of generality we select two representativeexamples of Ds(d) based on the packet delivery properties ofthe 802.11p subsystem in the JiST/SWANS simulation packagewith extensions from the University of Ulm. We use twoscenarios to illustrate our analytical model:• No congestion: A scenario with two vehicles commu-nicating in a 802.11p channel with default settings.No other vehicles introduce noise or packet loss inthis scenario. Only signal propagation effects reducepacket delivery success over longer distances betweenthe two vehicles.• High congestion: A scenario that uses a backdrop ofa high load on the communication channel, producedby vehicles using the same communication pattern asthe sender and receiver vehicles under investigation,i.e. NoOm or a variant of POoC. By projecting a1 km2 scenario with 260 vehicles on a√2 kmone-dimensional line we replicate a simplified highcongestion scenario as suggested in [7]. We extendthe scenario in each direction by the length of the max-imum sensing range with the same vehicle density toensure equal congestion in the center of the scenario.Figures 1 and 2 show the packet delivery rates for theselected example scenarios. Each certificate omission schemeresults in different channel congestion due to differing lengthsof messages causing some diversification. Figures 3 and 4show polynomial curve fitting applied to the simulated val-ues to arrive at smooth distribution curves. This representsthe baseline for packet delivery success (NPL only) in ourillustrative examples.The delivery rate function Ds(d) and thus the NPL modeldo account for the effect of payload sizes in a scenario.This implicitly includes the effect of the selected certificateomission protocol on channel load. Next we want to derive theexplicit probability of successfully receiving a message with anincluded certificate. We derive this from the overall messagedelivery success rate multiplied by the rate of certificateinclusion, which we introduce as the discrete value c. 0 20 40 60 80 100 100  200  300  400  500  600Packet Delivery Rate [%]Distance [m]NoOmPOoC2POoC3POoC4POoC5POoC6POoC7POoC8POoC9POoC10Fig. 1. Packet delivery rates without load on the communication channel 0 20 40 60 80 100 100  200  300  400  500  600Packet Delivery Rate [%]Distance [m]NoOmPOoC2POoC3POoC4POoC5POoC6POoC7POoC8POoC9POoC10Fig. 2. Packet delivery rates with high load on the communication channelDs(d) ∗ c (1)For example, in the case of POoC3 we omit the certificatetwo times followed by one inclusion, leading to an inclusionrate of 1/3. Figures 5 and 6 show the resulting graphs forthe example scenarios and the investigated omission schemes.The graph of NoOm is unchanged compared to the previousfigures, as the certificates are always included.As the probability of receiving a certificate is known, wecan derive the probability of reception of a certificate withina given number of beacon periods. This defines the extent ofCPL that is introduced by certificate omission. We considerthe probability of receiving a certificate within any of up to nbeacon periods as the inverse of the probability to not receiveany certificate within n beacon periods. We start with theprobability of not receiving a certificate within one beaconperiod, which is1−Ds(d) ∗ c (2)20114 IEEE Vehicular Networking Conference (VNC)42 0 20 40 60 80 100 100  200  300  400  500  600Packet Delivery Rate [%]Distance [m]NoOm fitPOoC2 fitPOoC3 fitPOoC4 fitPOoC5 fitPOoC6 fitPOoC7 fitPOoC8 fitPOoC9 fitPOoC10 fitFig. 3. Polynomially fitted packet delivery rate without load on thecommunication channel 0 20 40 60 80 100 100  200  300  400  500  600Packet Delivery Rate [%]Distance [m]NoOm fitPOoC1 fitPOoC2 fitPOoC3 fitPOoC4 fitPOoC5 fitPOoC6 fitPOoC7 fitPOoC8 fitPOoC9 fitFig. 4. Polynomially fitted packet delivery rate with high load on thecommunication channelTaken to the power of n, we get the probability that nocertificate is received in any of n beacon cycles. The inverseof this is then the probability that a certificate is included oneor more times in n beacon cycles. Thus we get the probabilityof having received a certificate within n beacon periods as1− ((1−Ds(d) ∗ c)n) (3)We now have two free variables d and n. To show anexample as a two-dimensional graph we fix d = 300. Theresulting graphs for the probabilities of receiving a certificatewithin n beacon periods are shown in Figures 7 and 8Finally, we can now combine the probability of havingreceived a certificate (CPL) with the probability of receiving apacket at all (NPL). The multiplication of these probabilitiesgives the overall probability of successful packet transfer underconsideration of both sources of packet loss.(1− ((1−Ds(d) ∗ c)n)) ∗Ds(d) (4) 0 20 40 60 80 100 100  200  300  400  500  600Certificate Delivery Rate [%]Distance [m]NoOm fitPOoC2 fitPOoC3 fitPOoC4 fitPOoC5 fitPOoC6 fitPOoC7 fitPOoC8 fitPOoC9 fitPOoC10 fitFig. 5. Certificate delivery rate without load on the communication channel 0 20 40 60 80 100 100  200  300  400  500  600Certificate Delivery Rate [%]Distance [m]NoOm fitPOoC2 fitPOoC3 fitPOoC4 fitPOoC5 fitPOoC6 fitPOoC7 fitPOoC8 fitPOoC9 fitPOoC10 fitFig. 6. Certificate delivery rate with high load on the communication channelFigures 9 and 10 show the overall packet delivery ratesfor the two sample scenarios, at the reference distance of 300m between sender and receiver. In Figure 9 we see how allschemes converge to the same ideal packet delivery successrate if there is no congestion. Certificate omission in this sce-nario only introduces down-sides without any improvements.Figure 10 however shows more diverse results. Certificateomission schemes converge to different packet delivery ratesafter the initial probability of CPL subsides. The speed ofreducing CPL and the convergence to higher overall packetdelivery success values give certificate omission schemes anadvantage over not performing certificate omission.IV. CONCLUSION AND FUTURE WORKIn this work we developed an analytical model to predictpacket delivery probabilities for secure broadcast communi-cation in vehicular networks under consideration of crypto-graphic packet loss. The results are in line with simulationmodels that have served as validation for the introduction ofomission schemes in previous works.20114 IEEE Vehicular Networking Conference (VNC)43 0 20 40 60 80 100 0  10  20  30  40  50Probability of certificate reception [%]Beacon periodsNoOm at 300mPOoC2 at 300mPOoC3 at 300mPOoC4 at 300mPOoC5 at 300mPOoC6 at 300mPOoC7 at 300mPOoC8 at 300mPOoC9 at 300mPOoC10 at 300mFig. 7. Probability of certificate reception after n beacon periods withoutload on the communication channel at 300m distance 0 20 40 60 80 100 0  10  20  30  40  50Probability of certificate reception [%]Beacon periodsNoOm at 300mPOoC2 at 300mPOoC3 at 300mPOoC4 at 300mPOoC5 at 300mPOoC6 at 300mPOoC7 at 300mPOoC8 at 300mPOoC9 at 300mPOoC10 at 300mFig. 8. Probability of certificate reception after n beacon periods with highload on the communication channel at 300m distanceThe model in this paper only considers the NoOm andPOoC omission schemes. Alternative omissions schemes, suchas CbCO and NbCO, rely on context sensitive mechanisms.Building models for such schemes remains as future work.Furthermore the model can be enhanced by considering vari-ations of relevant attributes such as variable payload length inmore detail. The availability of precise analytical models foromission schemes is expected to enable selection of schemesand parameters with the most beneficial attributes.ACKNOWLEDGEMENTThis work received funding from the European Union’sSeventh Framework Programme project PRESERVE undergrant agreement no. 269994.REFERENCES[1] ETSI TC ITS, “ETSI TS 103 097 v1.1.1 - intelligent transport systems;security header and certificate formats,” Standard, TC ITS, 2013. 0 20 40 60 80 100 0  10  20  30  40  50Probability of verifiable packet reception [%]Beacon periodsNoOm at 300mPOoC2 at 300mPOoC3 at 300mPOoC4 at 300mPOoC5 at 300mPOoC6 at 300mPOoC7 at 300mPOoC8 at 300mPOoC9 at 300mPOoC10 at 300mFig. 9. Overall packet delivery rate considering CPL and NPL without loadon the communication channel at 300m distance 0 20 40 60 80 100 0  10  20  30  40  50Probability of verifiable packet reception [%]Beacon periodsNoOm at 300mPOoC2 at 300mPOoC3 at 300mPOoC4 at 300mPOoC5 at 300mPOoC6 at 300mPOoC7 at 300mPOoC8 at 300mPOoC9 at 300mPOoC10 at 300mFig. 10. Overall packet delivery rate considering CPL and NPL with highload on the communication channel at 300m distance[2] M. Feiri, J. Petit, and F. Kargl, “Congestion-based Certificate Omissionin VANETs,” in Ninth ACM International Workshop on Vehicular AdHoc Networks (VANET ’12), June 2012, pp. 135–138.[3] ——, “Evaluation of Congestion-based Certificate Omission inVANETs,” in Fourth IEEE Vehicular Networking Conference (VNC ’12),November 2012, pp. 101–108.[4] M. Feiri, J. Petit, R. K. Schmidt, and F. Kargl, “The impact of security oncooperative awareness in VANET,” in Fifth IEEE Vehicular NetworkingConference (VNC ’13), December 2013, pp. 127–134.[5] G. Calandriello, P. Papadimitratos, J.-P. Hubaux, and A. Lioy, “Onthe Performance of Secure Vehicular Communication Systems,” IEEETransactions on Dependable and Secure Computing, vol. 8, no. 6, pp.898–912, nov.-dec. 2011.[6] E. Schoch and F. Kargl, “On the efficiency of secure beaconing inVANETs,” in Third ACM conference on Wireless network security (WiSec’10), March 2010, pp. 111–116.[7] R. K. Schmidt, A. Brakemeier, T. Leinmüller, F. Kargl, and G. Schäfer,“Advanced carrier sensing to resolve local channel congestion,” inProceedings of the Eighth ACM international workshop on Vehicularinter-networking. ACM, 2011, pp. 11–20.20114 IEEE Vehicular Networking Conference (VNC)44

English

https://ris.utwente.nl/ws/files/5455600/2014-12-vnc-feiri.pdf

Formal model of certificate omission schemes in VANET

Abstract

Similar works

Full text

Available Versions

NARCIS

University of Twente Research Information